Traditional baby bottles used for bottle feeding an infant were constructed from glass, or later on, plastic. A cap and nipple assembly were constructed to mate in threading engagement with an open mouth on the bottle. Prior to using them, it was necessary to sterilize the bottle, cap, and nipple by boiling them in water, which greatly increased the time required to prepare and fill the baby bottle in response to a hungry infant's cries. Furthermore, in use, the liquid removed by an infant from these bottles during feeding will create a vacuum inside the bottle, which eventually pulls air inside the bottle through the nipple opening. As a result, a feeding infant will frequently ingests air through the bottle nipple during feeding, which results in discomfort to the infant, and indirectly to the parents.
Subsequent improvements made to baby bottles have resulted in nursing bottles of a type having a liquid holding collapsible sack which is carried inside a protective cylindrical housing. The sack has an open top end with a sealed bottom which is inserted within the housing, afterwhich a terminating edge portion of the top end of the sack is folded over the housing end. The sack is held in this position and liquid is poured into the sack while it is suspended in the cylinder, afterwhich a threaded cap including a nipple is threadingly mated to an outer diameter end threaded portion of the housing which traps and seals the sack with the cap and housing. However, during this procedure it is virtually impossible to avoid sealing air inside the assembled nursing bottle. Even when an attempt is made to totally fill the sack, an air gap is still formed during attachment of the cap to the housing which traps air within the sealed-together sack and cap. Additionally, during extended periods of non-use when the nursing bottle is put down, or stored in a vertical position gravity pulls on the liquid in the sack, which creates a vacuum acting against the nipple. The vacuum pulls air through a feeding opening in the nipple into the sack. Therefore, it becomes necessary for the above reasons to manually expel air from within the sack by applying pressure to the bottom of the sack. Typically, a user inserts their fingers through a bottom opening in the housing in order to pressurize the sack and expel any air at the top of the sack through the feeding opening in the nipple. However, for many users, especially ones with short fingers, this finger-expelling procedure is cumbersome and ineffective, and often leads to split liquid or ineffective expulsion of air.
Accordingly, a number of devices have been developed in an effort to facilitate easier removal of air from these collapsible-sack-containing nursing bottles. U.S. Pat. No. 4,796,767 discloses a pressure applicator for use in expelling air from a flexible liner within a nursing bottle. However, the '767 device requires two-handed use, one for holding the nursing bottle, the other for holding and inserting the applicator within the bottom of the bottle. For many users, two-handed use is inconvenient at best since infants typically cry when they are hungry, thereby necessitating a parent or caretaker hold the infant in order to comfort and quiet them while at the same time preparing a nursing bottle. Therefore, difficulties arise when one fills and prepares a nursing bottle while concurrently comforting an infant.
U.S. Pat. No. 5,109,996 discloses a plunger body designed for insertion within a nursing bottle that prevents the liquid filled sack from filling up with air as the bottle sits during interrupted feeding of an infant. A cap can be affixed to an open bottom end of the plunger to enclose a cavity within the plunger body in which articles, for example spare sacks, can be stored. However, the plunger body of this device by necessity is sized in proportion to the inside dimensions of a nursing bottle. Therefore, the width of the base in relation to the height of the plunger is very small, making the plunger very unstable and unsuitable for use as a self-supported device during single-handed operation.
U.S. Pat. No. 5,356,016 discloses a nursing bottle having an open bottom cylindrical body into which a pair of opposed longitudinal slots are formed for supporting a longitudinally movable internal plunger body. The plunger body has a pair of tab handles extending through the slots which are accessible by a user during use to allow movement of the plunger longitudinally within the body in order to compress a liquid filled sack carried within the body and expel any air therefrom. However, movement of the plunger body requires two-handed operation, the bottle is made from an increased number of parts, and the plunger body complicates the assembly and cleaning of the bottle.